1. Field of Invention
The invention relates to an improved apparatus for the contact free disintegration of a calculus located in the body of a human being. In particular, the invention provides improved transducer and lens combination for generating a focussed converging shockwave having sufficient energy to disintegrate the calculi located in the body.
2. Prior Art
Extracorporeal shock-wave lithotripsy is a medical procedure that disintegrates renal (kidney) stones. The term, lithotripsy is derived from the Greek and literally means "stone crushing." This medical "stone crushing" technique has been demonstrated to be effective on renal (kidney), upper ureteral and biliary stones (gallstones) in human patients. The technique is noninvasive and eliminates the need for the more conventional stone removal procedures, such as open surgery.
Stone crushing is achieved by the geometric focussing of energy, such that the area of focus coincides with the stone that is to be shattered. The focussing of energy is a principle that is frequently used to obtain a strong effect within a confined area. The application of focussed energy is effective on the stone, since it is relatively brittle. The application of stress leads to the rapid formation of cracks and eventually to the disintegration of the stone.
Although a single intense pulse will generally shatter the stone, a pulse train of lower intensity and short duration pulses will generally produce smaller and more uniform particles. One drawback to a series of pulses is that during treatment the small particles produced during the fragmentation shock waves may obscure the larger remaining fragments. This may result in the shielding of the larger, remaining fragment from the shock wave, which results in an increase of treatment duration.
On the other hand, the accumulated small particles may lead the care provider into prematurely terminating treatment before completion, due to the inability to accurately view the remaining larger fragments.
Another problem associated with the production of small particles, is that gravitational forces may cause the small particles of settle into the lower calyces, thereafter requiring an excessive amount of time to effectuate clearing these lower calyces.
Despite the problems associated with the smaller particles, for the kidney stone patient, there are number of direct benefits from the lithotripsy technique that outweigh these minor disadvantages. The technique is capable of reducing the length of hospital stay from 7 to 14 days for conventional stone removal surgery to 4 days or fewer. It also eliminates the 4 to 6 weeks post-hospitalization convalescence commonly required after major surgery. Typically, the kidney stone patient will naturally and spontaneously pass the stones in the urine, much as if the stones were naturally grain-sized. However, after fragmentation, there will be many more grain-sized stones than occur naturally.
There are several other techniques presently used to treat kidney stones, as an alternative to major surgery. For example, if the stone remains grain-sized, it usually passes spontaneously in the urine and the patient can be treated with drugs to reduce the pain and to prevent future stone occurrence. However, this technique is usually only effective for very small stones.
Depending on the size and the location of the stone, several other clinical and surgical methods are available. If the stone has formed or lodged itself in the lower urinary tract and bladder, a standard cystoscope combined with a stone basket or a special stone removal forceps may be used to extract the stone. During the procedure, a local anesthetic must be administered to minimize the pain. Additional drawbacks are the possibilities that the ureter will be damaged and the formation of additional stones that would prohibit the repetition of the procedure.
Ultimately, if the stone cannot be removed by any of the conventional methods, the patient must resort to open surgery. The traditional surgical procedure is a major operation that requires an incision into the kidney or ureter to remove the stone. In addition to the normal morbidity and risks associated with major surgery, another stone could form, necessitating further surgery. Furthermore, there is a chance that the patient will finally lose the kidney altogether.
As an alternative to open surgery, lithotripsy was explored. However, early lithotriptic treatment employs invasive ultrasonic lithotripsy. The lithotriptic means employed involved an insertion of an ultrasonic probe into a small incision in the patient's side. While this required less recovery time than traditional surgery, it was still surgery, however small the incision and furthermore carried all the inherent risks associated with surgery.
Noninvasive extra corporeal shockwave lithotripsy was developed and overcame the negative risks of surgery. A shock wave is generated and ultimately focussed at the stone. The focussed wave strikes the stone, it disintegrates the stone and the stone fragments are ultimately passed through the ureter.
Early versions of extra corporeal (EC) shock wave lithotripsy required the patient to be immersed in a large tank of water. Originally, a spark gap electrode was fixed at one end of a large stainless steel tub and generated a shock wave as the electrode was discharged underwater. Since the impedance properties of water and soft tissue are similar, the shockwave entered the body without damaging the soft tissue. If care is taken to direct the shockwaves at the calculi, other parts of the body are generally not affected. Typically, bones are not affected by this procedure due to their high tensile strength and the brevity of the pulses. However, the large difference in acoustical impedance between the water and the stone results in enough pressure to shatter the stone. The process was painful and the patient generally required an epidural or general anesthesia.
Before treatment could commence, it was necessary to locate and identify the stone. Prior to immersing the patient into the large stainless steel tank of water, the patient was x-rayed thus prelocalizing the stone. After every 100 pulses or so, the patient was lifted out of the tank and x-rayed again. This was to ascertain the status of the stone and to verify the effectiveness of the treatment. With an average of 1600 pulses, the patient could be x-rayed as many as 17 times to verify that the stone was fragmented and to confirm the completeness of the treatment. Although an x-ray produces a sharper and clearer progress report, the patient was required to undergo a number of detrimental radiation exposures.
Another problem associated with immersing the patient is the inability to accurately reposition the patient once the patient had been moved. Since the electrode is fixed, the patient must be positioned and repositioned. This led to inaccuracies and sometimes painful results.
An alternative localization technique requires the use of two x-ray images, with the focal point at the central point of the two screens. Typically, the two x-ray devices for locating the stone are disposed next to the shockwave generator and reflector and are on an opposite side or axis of the generator. The two central beams of the two x-ray heads intersect the axis of the reflector running through the two focal points. The intersection should be near or at the location of the stone. Establishment of a relationship between a spatially fixed mark and the patient is required. The equipment generally is repositioned vis-a-vis the patient.
From a point of view of expense, the utilization of two x-ray heads and devices is relatively expensive to operate and to maintain. There is also the question of the radiation dosage that the patient receives.
When using an x-ray procedure, the patient must be positioned or repositioned such that the stone is located at the focal point of the shock wave. For example, when using a stainless steel water tank, the patient is then positioned with a hydraulic system so that the calculus is at a predetermined focal point. While this allows access to a more accurate target region, it is necessary to physically reposition the patient for each image. Disadvantageously, the patient is required to remain immobilized during the location procedure.
Ultrasound imaging is another suggested alternative means of locating and monitoring the stone. Generally for well-defined stones, ultrasound is sufficient to monitor the progress of treatment. Furthermore, the use of ultrasound eliminates the problems of radiation dosage that arise with the use of x-ray localization techniques.
Another advantage of ultrasound imaging is the ability to monitor real time progression of the treatment. Additional ultrasound monitors can be positioned within the treatment system in such a way as to virtually eliminate repositioning of the patient.
However, ultrasound imaging does not provide the same same high contrast results or "pictures" that an x-ray unit can provide. Since the resulting images are not as sharp and have as high a contrast, which can be due to the limitations to the technique itself, the size and quantity of the stones, or the size of the patient, can make progressive monitoring of patient and the subsequent treatment somewhat risky and haphazardous.
Furthermore, prior to the elimination to the large steel water tank, the use of ultrasonic monitoring equipment was severely limited, as the equipment had to be located within the vicinity of the patient itself.
A combination of the two monitoring techniques, that is x-ray and ultrasound, would be suited except for the problem of patient repositioning. Immobility of the patient is essential and are identical to the problems discussed previously in connection with the x-ray techniques.
The eventual elimination of the large stainless steel water tank resulted in a more compact and effective system. Furthermore, elimination of the water tank also gave rise to the possibility of using ultrasound imaging. Patient immersion became unnecessary as the acoustic waves were still propagated through a water containment bag rather than through an open body of water. The patient is positioned over a cushion or a bag filled with water. The cushion is coupled to a stretcher and to the patient by a layer of ultrasound gel.
Once the patient positioning had been improved, the means for producing the shock wave needed to be improved. For example, the prior method of producing focussed acoustic waves required an electric discharge generated across an underwater spark gap, positioned in the focus line of an elliptical reflector. The early spark gap systems generated a shockwave in a large tank of water, in which the patient had previously been positioned. The process shattered renal stones by a brute force method. The treatment required the patient to have an epidural, spinal or general anesthesia in order "to keep the patient under control, to avoid hurting the patient and to aim better at the stone."
Improvements made it possible to remove the patient from the water tank and to virtually eliminate the need for an epidural or general anesthesia. Generally, a larger ellipsoid for focal precision and a decrease in the power output are credited for the improvements. Accuracy improved as the focussing process was computer controlled, rather than visual sighting by the operator. The computer automatically positions the patient in the shockwave focus by controlling the patient's table.
The spark gap systems create a diverging pulse or explosion. The energy produced is distributed over a relatively large area, for example 15-20 square mm and requires that the energy be focussed within a parabolic reflector. This diverging pulse induces arhythmia, pulverizes tissue and causes bruising. To avoid the inducement of extrasystoles, the shockwave must be released as a function of ECG or respiratory triggering. Furthermore, the spark gap electrodes tend to be somewhat costly.
On the other hand, an alternative means using a piezoelectric transducer generates a converging pulse and eliminates the need for cardiac or respiratory triggering. With a converging pulse system, an area of impact of approximately 2-3 square mm is observed. This results in less tissue damage with approximately the same amount of pressure. Generally, the patient only requires a local or a topical anaesthetic, if any at all.
The piezoelectric crystal is the basis for the piezoelectric transducer. A piezoelectric crystal is a piece of natural quartz or other asymmetric crystalline material capable of demonstrating the piezoelectric effect. Piezoelectricity is a phenomenon first noticed in 1880 whereby certain crystalline substances generate electrical charges when subjected to mechanical deformation. The reverse effect also occurs, that is, a voltage applied across the crystal causes mechanical deformation or flexing.
To produce the shock wave, a piezoelectric crystal is deformed by applying a high frequency, high voltage pulse of the proper polarity, thus causing the crystal to compress. The voltage is then withdrawn from the crystal, such as shorting the crystal to ground and the crystal expands to its "normal" state. This results in a pressure wave front that propagates through a medium such as water.
Typically, the shock wave is focussed by arranging piezoelectric crystal elements in a mosaic pattern on the surface of a dish generally shaped as a concave lens. The pattern and lens may be designed such that each individual element is excited by a pulse generator simultaneously. Thus, the waves produced by the crystals arrive at the target area focussed and in phase, with a very narrow near ideal shape and with high energy at the focal point. Advantageously, the dish is shaped to produce very small focal areas in which the energy is sufficient to destroy the stone but virtually painless.
Another advantage of the piezoelectric system is the ability to locate an ultrasonic probe in the center of the focussing dish and permits real time monitoring.
A major disadvantage of the multi-element piezoelectric system is the necessity of simultaneously triggering 300 to 3000 elements on the concave surface. One method currently used triggers the elements with individual power supplies. An alternative method uses a single power source to simultaneously trigger all the elements.
Another disadvantage is the requirement that the numerous elements be specially made. This decreases the cost effectiveness of the piezoelectric system.